The present invention is generally related to a bias circuit which may provide a biasing voltage to biased circuit, and more particularly, is related to a bias circuit that may provide a biasing voltage that varies accordingly to changed characteristics of the biased circuit due to process variations.
FIG. 1 illustrates a distributed amplifier and a passive gate bias. The distributed amplifier is formed on a wafer, whereas the passive gate bias is formed on a circuit board which receives the distributed amplifier chip. Since the distributed amplifier is formed on a wafer, the distributed amplifier may have characteristics which may vary because of wafer lot to wafer lot variations. One such variation may be quiescent points of transistors which make up the distributed amplifier. To compensate for this variation, oftentimes, the passive gate bias is connected to the distributed amplifier. In the circuit shown in FIG. 1, the distributed amplifier receives an input signal at gates of the transistors. If the distributed amplifier was formed as designed and behaves as designed, then the input signal may not need to be biased. However, due to the wafer lot variations and the resulting differing characteristics in the amplifier, the input signal may need to be biased such that the transistors of the distributed amplifier operate at its quiescent point. As such, the passive gate bias is applied to the gates of the transistors to apply the biasing voltage to the input signal. In manufacturing, the distributed amplifier is tested during assembly to determine the amount of the wafer lot variation, and then the appropriate passive gate bias is selected to compensate for the variation. The passive gate bias shown in FIG. 1 is a resistive voltage divider and in this respect, appropriate resistor values are selected to compensate for the wafer lot variations. The purpose of the resistive voltage divider is to bias the input signal to the distributed amplifier such that the distributed amplifier may operate at its quiescent point.
This process of selecting proper resistors is very time consuming and inefficient in that at the least a statistical sampling must be done on the distributed amplifier of a particular wafer lot such that proper resistors are selected for the resistor voltage divider. Additionally, this process relies on a person to make a repetitive process which creates the possibility of error.
Accordingly, there is a need for an improved bias which may account for the wafer lot to wafer lot variations. As will be shown, an aspect of this invention is to design an active bias which is formed on the wafer which the distributed amplifier is formed on to thereby impart the wafer lot variations onto both the distributed amplifier and also the active bias.